It is widely known to use nitric oxide (NO) in a variety of applications. Next to technical applications such as an intermediate in the Ostwald process for the synthesis of nitric acid from ammonia, especially several therapeutic applications using nitric oxide are known.
One of the most famous therapeutic applications of nitric oxide is the administration for neonates suffering from Persistent Pulmonary Hypertension (PPHN). However, many comparable or other therapeutic applications are known and discussed for the use of nitric oxide. As an example, nitric oxide is used by the endothelium of blood vessels to signal the surrounding smooth muscle to relax, thus resulting in widening the blood vessels and therefore increasing blood flow. This leads to nitric oxide being particularly applicable for the therapy of hypertension. Further exemplary applications for nitric oxide are directed towards improving lung function and treating or preventing bronchoconstriction, reversible pulmonary vasoconstriction, or for treating or preventing arterial restenosis resulting from excessive intimal hyperplasia. Apart from that, the administration of nitric oxide is particularly useful for treatment of infected tissue e.g. to kill bacteria. This application mostly involves topical delivery of a source of nitric oxide containing gas to a skin surface containing infected tissue.
The storage of nitric oxide for example in containers, or gas cylinders, respectively, may however cause difficulties. Due to the fact that nitric oxide tends to react with oxygen, even minor impurities of oxygen in the stored gas may cause the formation of nitrogen oxides in higher oxidation states, in particular the formation of toxic nitrogen dioxide (NO2). Therefore, nitric oxide has only limited useful life time and may thus be stored only in a low concentration and for a limited time. For many applications, it is therefore preferred to generate nitric oxide in situ, i.e. directly before use.
The formation of nitric oxide may additionally lead to problems because of which several attempts to form nitric oxide are exercised. As an example, it is known to generate nitric oxide by using light being radiated on a chemical liquid leading to nitric oxide being evaporated. Further attempts are based on the formation of nitric oxide starting from air, for example by using a gas discharge. A gas discharge in air, however, generates not only nitric oxide, but also other compounds which bear health risks, e.g. ozone (O3) and nitrogen dioxide (NO2) may be formed.
A further attempt for generating nitric oxide is known from U.S. Pat. No. 6,238,941. Here, the main focus lies in the decomposition of nitrous oxide (N2O) with a very high conversion efficiency. Starting from nitrous oxide as a precursor, nitric oxide is generated in particular by a thermal decomposition of nitrous oxide forming a nitric oxide comprising gas. The method is carried out without the use of a catalyst thereby applying high temperatures. The reacted gas mixture is then cooled by heat exchange. The reacted gas mixture comprises nearly no nitrous oxide but nitrogen as main component with a big amount of nitrogen oxides (NOx).
One of the major drawbacks of the known methods for generating nitric oxide is the considerable formation of nitrogen oxides in higher oxidation states, in particular the formation of nitrogen dioxide. Due to the high toxicity of these nitrogen oxides in higher oxidation states, especially nitrogen dioxide, the generated gas mixture cannot be used directly, but further purification steps are required instead, in particular for therapeutic applications.